Standard 'Watch' or Real-Time-Clock crystal. 32.768kHz means it is precisely half of a 16-bit counter. Start counting a 0x80 (or 32768). When the counter rolls over from 65535 to 0, then you know that exactly one second has passed. Reset the interrupt, reset the counter, and start counting again! Rated at 12pF load capacitance and +/- 20ppm stability. Small cylinder package.
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Documents: CFS206 Datasheet